Nowadays, radiation of highly varied types, such as alpha, beta and gamma radiation, or particles, such as electrons, neutrons, ions or nuclei, are used for the analysis of material samples. Radiation analyses of this type are implemented for example in the localisation and identification of dangerous substances, for example concealed explosives, but also in the field of medicine in combination with imaging methods, in radiation measurement or in research on the structural analyses of materials. In this case, radiation detectors which make it possible to establish the direction of incidence or the origin of the radiation to be detected and evaluation methods for identifying the materials present in the respective radiation source are in particular required.
In the methods used to date, incident radiation to be detected was initially collimated by hole or lamella collimators in front of the actual detector material. Usually, a hole collimator of this type is formed from a plate of a material with a high atomic number, for example lead, tungsten, iron or brass, which plate is provided with a plurality of holes of a small diameter. In this way it is possible for example for only the gamma rays passing through the corresponding collimator subsequently to be recognised by a detector.
In this case a low sensitivity of the detector is a drawback, in particular at high radiation energies, because only the gamma rays which are incident parallel to the axes of the holes or at a small aperture angle are to be used in the evaluation. Therefore, the wall thickness between the collimator holes must be selected as a function of the collimator material, collimator thickness, hole diameter and photon energy. In particular for photons with energies of more than 200 keV, the necessary wall thickness between the collimator holes may exceed the diameter of the holes. The transmission, i.e. the cross-sectional area of all collimator holes in relation to the total face area (front face) of the respective detector, will then be low. The absorbing cross-sectional area then actually exceeds the penetrable surface area of the collimator. The higher the energy of the particles or radiation to be detected, the more unfavourable the transmission becomes and the lower the probability of the respective detector responding. This applies in particular to high energy gamma radiation. Alongside the aforementioned collimator-based detectors, detection methods which detect direction without collimation are also known, such as the Compton camera or the positron camera in positron emission tomography (PET).
The principle of the Compton camera is that instead of the aforementioned mechanical collimation, an electronic collimation is performed to determine the direction of radiation incident on the corresponding detector system. A radiation photon incident in a Compton camera thus initially passes through a first scattering detector in which Compton scattering takes place. If the scattering detector is configured in a granular manner, the location of the interaction will be detected. In a second absorption detector at a distance, the scattered photon is completely halted, it being possible to record the location and the energy. If the two scatterings in the scattering detector and absorption detector are shown to be coincident, then the position of the radiation source which emitted the photon can be reconstructed on a cone surface with the apex at the location of the scattering detector. The location of the radiation source can be determined by means of suitable tomographic reconstruction algorithms. This principle is primarily used in astrophysics for the measurement of gamma radiation sources with photon energies of up to 30 MeV.
A drawback is that the scattering detector must be very thin so that at most one Compton interaction takes place therein. This substantially reduces the probability of an interaction in the detector. Furthermore, complete energy deposition of the photon energy in the absorption detector must be ensured, and in real detector systems this is always achieved only for a fraction of the incident photons. This reduces the efficiency of the Compton camera.
Locating explosives, for example landmines in a cleared area or concealed explosives in luggage or cargo, is a particular field of application of detectors which use bearing resolution.
Explosives generally have a high concentration of nitrogen compounds. The recognition of these nitrogen accumulations must generally take place in the environment of other lighter atoms, such as organic compounds and plastics material, sand or earth, which are part of the soil in the case of landmines or which form the main component of the container to be analysed. These materials primarily comprise light elements, such as carbon (C), nitrogen (N), oxygen (O), fluorine (F), silicon (Si), magnesium (Mg) etc., which emit high energy gamma quanta or gamma radiation under neutron excitation (activation with neutrons). Conventionally, for example at airports, transmission X-ray devices are used for luggage inspection. However, no element-specific signal is detected in this way, explosive devices being detected only on the basis of their shape in the X-ray image.
X-ray backscattering detectors and methods which employ neutrons are proposed for the detection of landmines in the ground, for example in “Detection of buried landmines and hidden explosives using neutron, X-ray and gamma-ray probes”, G. Nebbia and J. Gerl in europhysics news, July/August 2005, pages 119 ff. In this case, backscattered neutrons or gamma quanta from the neutron capture, for example, are measured with scintillation counters. This generally requires high-volume detector assemblies which must be moved close to the ground. This does not allow the detection of the direction of the gamma source, produced for example by neutron activation, relative to the detection system above the ground.
A further difficulty is the identification of individual emission lines, which are characteristic for example of nitrogen or other material or element compositions, from a measured intensity spectrum in order to make predictions concerning the composition of the respective material samples and to detect the presence of explosives. Disadvantageously, this requires energy-resolving spectroscopy with a high energy resolution. This involves a high technical expense and leads to lower reliability of the relevant detector.
An object of the invention is therefore to determine the presence of predetermined materials from the detected radiation. A further object of the invention is to provide a detector assembly which allows improved detection and evaluation of radiation.